Current processes for making pigments useful in coloring ceramics, plastics and glass require calcination of the reactants, usually metal oxides, at high temperatures for relatively long periods of time. The calcination process is then followed by a grinding process to produce a powder fine enough to be carried by the vehicle. It would be advantageous if a method for making pigments could be devised which did not require long reaction times and high temperatures, and/or a separate grinding step.
Processes for the production of zirconium-silica based pigments using high temperature calcination of oxides in the presence of a chromophore and a mineralizer component are described in a number of publications. For example, U.S. Pat. No. 3,510,332 outlines a process for the production of zirconium-praseodymium yellow pigments from zirconium silicates without separating sodium silicate. The first step is the decomposition of zirconium-silicate by heating an alkali-zirconium silicate mixture to temperatures of 800.degree.-1200.degree. C. In the second step, an aqueous slurry of the decomposed product is formed by mixing the decomposed product with water in a molar ratio of water to zirconium silicate of from about 5:1 to 20:1, and with a praseodymium compound in an amount, sufficient to form the yellow pigments. The third step involves adding a mineral acid to the aqueous liquid slurry in an amount, sufficient to neutralize at least 0.5 alkali equivalents, thereby solidifying the aqueous slurry. The fourth and final step is calcining the solidified mixture at a temperature of from 850.degree. to 1300.degree. C. This patent employs calcination steps which are conducted at high temperatures for relatively long periods of time, which is undesirable for pigment manufacture.
Pigments may also be prepared by calcining mixtures of zirconium and silicon salts in the presence of a chromophore and a mineralizer component. Yellow ceramic pigments containing praseodymium using 1 mole ZrO.sub.2, 0.8-1.4 mole SiO.sub.2, 0.06-0.04 mole flouride, 0.1-3.5 mole chloride, 0.1-0.4 mole Pr and with water equal to 0.5-10 times the moles of chloride employed are described in WPI/Derwent Abstract 78-23485A/13 of DE 2642143. The Abstract states that water may be added as such, or be present in the compounds used in the mixture, which may also contain 0.01-0.1 mole of an alkali nitrate per mole chloride. The mixture is preferably calcined 0.5-3 hours at 820.degree.-980.degree. C. and the resulting products are employed to manufacture glazes.
U.S. Pat. No. 3,589,925 teaches that zircon based ceramic pigments may be prepared by calcining mixtures comprising zirconium and silicon oxide (or compounds capable of yielding these materials on calcination) in the presence of a chromophore and one or more mineralizer components, the proportions of zirconium oxide (zircon) and silica in the mixture being such that zirconium silicate is formed on calcination. Blue and yellow pigments may be formed by calcining mixtures of the type described above in which the coloring agents are, respectively, vanadium pentoxide and praseodymium oxide and in which, in each case, mineralizer components comprising the sources of alkali metal ions, fluoride ions and either chloride or bromide ions are now employed. The discovery of the patent is that ceramic pigments may have still further enhanced color strength by calcining mixtures of the type described above, in which the mineralizer components include a source of barium ions, in addition to sources of alkali metal ions, fluoride ions and either chloride or bromide ions.
Yellow zirconium silicate ceramic pigment may be prepared by calcining a mixture of zirconyl hydroxide, quartz, praseodymium oxide and a mineralizer, according to the title of WPI/Derwent Abstract 79-003743/31 of RO 663383. Similarly, there is WPI/Derwent Abstract 79-72995B/40 of JP 79027767 which describes transfer paper giving a clear red color by heating at high temperature. On a transcription paper substrate are formed an adhesive layer and a yellow pigment layer containing vanadium and zirconium, and an additional pigment layer containing pink color pigment. It is not mentioned in the Abstract how the yellow pigment color is made. Nevertheless, it continues to be seen from these publications that calcining is considered a necessary step in pigment formation.
Zirconium dioxide-silicon dioxide-praseodymium oxide ceramic pigments have added thereto 0.01 to 10% of an oxide of antimony, niobium, bismuth and/or tantalum according to U.S. Pat. No. 3,899,347. Once again, however, calcining and grinding steps are necessary to achieve the pigment of this patent.
Another process requiring calcining is described in U.S. Pat. No. 3,756,840. It mentions the preparation of praseodymium yellow zircon-based pigments by calcining silicozirconate, zirconium sulfate, a mineralizer and praseodymium oxide at a temperature below 900.degree. C. The silicozirconate is present in a stoichiometric excess; e.g. 10-50 molar percent, with respect to the basic zirconium sulphate. The zirconium sulfate is preferably of formula (ZrO.sub.2).sub.2.SO.sub.3 and may be prepared by reacting the silicozirconate with sulphuric acid. Glazes containing the pigments are of superior quality to those containing pigments derived from smaller quantities of silicozirconate, it is stated.
U.S. Pat. No. 5,032,420 discloses a method for applying a yellow incandescent bugfoiler pigment coating comprising a fine-ground blend of praseodymium doped zirconium silicate and silica. This cadmium-free blend is preferably applied electrostatically in three coats to the interior of a bulb to diffuse the light and block the emitted UV wavelengths. The method of making the pigment is not disclosed.
Yellow ceramic pigment compositions with chemical resistance containing silicon, aluminum, calcium, sodium, potassium, cerium, praseodymium and zirconium dioxide are described in WPI/Derwent Abstract 78-34775A/19 of SU 560843. The yellow ceramic pigment composition comprises (in wt. %): SiO.sub.2 50.21-61.94; Al.sub.2 O 9.15-11.26; CaO 0.52-0.64; Na.sub.2 O 3.37-4.15; K.sub.2 O 1.74-2.14; CeO.sub.2 2.0-6.0; Pr.sub.2 O.sub.3 2.0-9.0; ZrO.sub.2. The addition of CeO.sub.2 and Pr.sub.2 O.sub.3 are defined as culturing agents and additional ZrO.sub.2 give yellow production with improved chemical resistance and reduced firing temperature (1050.degree.-1100.degree. C.). Preparation of the pigment is not further discussed within these publications, however.
U.S. Pat. No. 3,640,745 describes the preparation of agglomerated pigmentary titanium dioxide by vapor phase oxidation of titanium halide in a reaction chamber at elevated temperatures in the presence of controlled amounts of alkali metal cation and auxiliary gas. The production of superfine oxide powders by flame hydrolysis in a burner or by hydrothermal synthesis is described by G. W. Kriechbaum, et al. in "Superfine Oxide Powders--Flame Hydrolysis and Hydrothermal Synthesis," Angew. Chem. Ind. Ed. Eng. Advanced Materials, Vol. 28, No. 10, 1989 pp. 1416-1423.